In recent years, various proposals have been offered on a dynamic pressure bearing device for supporting various kinds of rotating bodies such as a magnetic disc, a polygon mirror and an optical disc which are rotated at a high speed. For example, in FIG. 13, there is shown a dynamic pressure bearing device mounted to a spindle motor for a hard disc driving device. Arrow marks in a solid line in FIG. 13 indicate directions along which pressures in a lubricating fluid in bearing portions work.
As shown in FIG. 13, a rotary shaft 110 is inserted in a bearing sleeve 100 in a freely rotatable manner. A lubricating fluid 120 such as oil is fed in a small clearance in a radial direction between an inner circumferential surface of the bearing sleeve 100 and an outer circumferential surface of a rotary shaft 110. Radial dynamic pressure bearing sections 130 and 131 at two sites spaced from each other are formed on the upper side and the lower side in the small clearance in the rotation axis.
A thrust plate 140 is joined to the rotary shaft 110. Both end surfaces of the thrust plate 140 in the rotation axis direction is arranged so as to face the bearing sleeve 100 and a counter plate 150 attached to the bearing sleeve 100 in the rotation axis direction with a small clearance interposed therebetween. The lubricating fluid 120 is fed in the small clearance so as to be in continuation from the radial bearing section 130.
Thrust dynamic pressure bearing sections 160 and 161 are formed on the top surface and bottom surface of the thrust plate 140 in the rotation axis direction. Besides, communication holes 170 are formed so as to cause the thrust dynamic pressure bearing sections 160 and 161 to communicate with each other. The lubricating fluid 120 is circulated to the radial dynamic pressure bearing sections 130 and 131 and to the thrust dynamic pressure bearing sections 160 and 161 through the communication holes 170.
The lubricating fluid 120, however, flows into a flow path 180 in the small clearance between the lower side radial bearing section 131 and the top surface thrust bearing section 160 from the communication holes 170 and collides with an inner circumferential corner portion 101 formed in the flow path 180 of the bearing sleeve 100. Hence, a flow of the lubricating fluid 120 is disturbed. Since no relief in space is available in the small clearance in the neighborhood of the inner circumferential corner portion 101 of the bearing sleeve 100, the intrinsic flow of the lubricating fluid 120 and a flow thereof created by collision thereof with the inner circumferential corner portion 101 of the bearing sleeve 100 are affected by each other. Therefore, a behavior of the lubricating liquid 120 is altered. As a result, vibrations are generated by causes of the collision of the lubricating fluid 120 with the bearing sleeve 100 and alteration in behavior thereof. This further creates a vibration problem of a spindle motor to which the dynamic pressure bearing device is mounted and a noise problem caused by the vibrations.